Containers used for the storage or shipment of spent nuclear fuel or other radioactive material must be designed to seal in the radioactivity under possible adverse conditions that might occur during use or transport. The container walls must provide adequate shielding to block radiation and also be thermally conductive to dissipate the heat generated by the radioactive material stored within the container.
The considerations in design of storage and shipping casks involve providing a sealed pressure vessel according to applicable codes, and providing adequate thermal conductivity for dissipation of heat generated by the contained radioactive material. These considerations require containers that are constructed predominantly of metals of high strength. Additionally the containers must provide shielding against escape of electromagnetic radiation such as gamma rays. In general, this type of shielding is provided by metal of suitable thickness, with high density materials being favored. Also, the containers provide shielding against escape of nuclear particles such as neutrons. In general, this type of shielding is provided to a limited extent by the thickness of the container or distance from radioactive source to the outer surface of the container. Often the shielding effectiveness is enhanced by use of neutron absorptive and/or moderator materials such as boron carbide or a material high in hydrogen such as various hydrocarbons in combination with materials having high neutron absorption properties.
Of special importance is the need to maintain the physical integrity of the container in possible accidents. To this end, tests have been devised such as dropping the container from a height of 9 meters on a non-yielding surface. Other tests involve dropping the container from a height of 1 meter on a mandrel of defined configuration. Tests are carried out typically at about 800.degree. C. for about thirty minutes. Various types of containers have been proposed that might approach satisfying the requirements of the above tests to varying degrees and other safety requirements that might be anticipated for extreme accident conditions.
For example, a simple design of a container for storage and shipment of radioactive material involves a cylindrical metal cup-shaped container with cover to provide an inner cylindrical cavity for the radioactive material. Such a container can be fabricated from steel by welding. It would have the required strength as a pressure vessel but would have inadequate shielding properties in reasonable thickness. Lead has the desirable properties of high absorption capability, low cost, and castability; but has low mechanical strength and a relatively low melting temperature.
Containers have been made with a laminated structure for shielding consisting of lead cast between inner and outer shells of stainless steel. Under high temperature test conditions representing possible conditions of exposure during use, there is a tendency of the lead intermediate layer to melt and flow from its proper place, resulting in the loss of its absorption effectiveness. The use of special insulation (e.g. moist plaster in the outer and intermediate layers) complicates the design and is of questionable effectiveness, particularly when the cavity contains spent nuclear fuel that can generate significant amounts of internal heat. The provision of forced coolant passages in the outer or inner shell, within the contents cavity, or the attachment of coolant tubes to the shell by welding, results in the container being susceptible to failure during the coolant cycle. Passive air-cooled designs are preferred. In addition to cooling at the outer cylindrical surface, additional connective air cooling can be provided such as by use of a centrally located annular chimney as in U.S. Pat. No. 3,111,586 of Rogers.
Other container designs have been proposed based on a cast iron shield as in U.S. Pat. No. 4,272,683 of Baatz et al. The shieId can be cast around a relatively thin inner layer of drawn or welded stainless steel placed in the mold, although it is mentioned that it might be advantageous to provide the inner protective layer by other means such as galvanic coating of the cast shield. Heavy metal particles (for enhanced gamma ray absorption) and channels (for adding neutron absorbing material) are incorporated at the time of casting the shield. The problem with a unitary cast iron shield is the susceptibility to brittle fracture and crack propagation through the entire shield. Safety considerations require that the shield remain in place even under extreme conditions resulting in fracture.
In general, the prior art of casting shields has emphasized maintaining the continuum of metal or integrity of the body, presumably based on heat transfer considerations without consideration of the adverse safety aspects of fracture propagation through the structure.